Fossil fuel burning, the main energy source for human activity, has caused environmental issues such as global warming, ozone layer depletion, and ecological devastation. Thus, the development of alternative energy sources is needed. A fuel cell, an electrochemical device in which the chemical energy stored in fuel is converted directly to electrical current through an electro-catalytic process, is a good candidate for such an alternative energy source. The striking characteristics of fuel cells include their low environmental pollution and high energy conversion efficiency in comparison to heat engines. For instance, water is the only generated product in a hydrogen-oxygen fuel cell. Further, fuel cells have low noise levels. Because of these advantages, fuel cells have been regarded as an excellent energy source for portable electronics, land vehicles, airplanes, submarines, etc., and have attracted more and more attention in the new energy technology development field. Currently, numerous research efforts are underway to improve the performance of fuel cells, including large power and energy density, cheap catalyst, long shelf life, and ease of miniaturization for portable electronic devices.
Among a variety of fuel cells, a proton-exchange membrane (PEM) fuel cell is one of the principal types. Direct methanol fuel cells (DMFCs) are a subcategory of PEM fuel cells in which methanol is used as the fuel. The strengths of DMFCs are the ease of transport of methanol, an energy-dense yet reasonably stable liquid under all environmental conditions, and the lack of complex steam reforming operations. However, efficiency is presently quite low for these cells, so they are targeted especially for portable applications, in which energy and power density are more important than efficiency. Indeed, the energy density of methanol is an order of magnitude greater than even highly compressed hydrogen, and 15 times higher than lithium-ion batteries. Military applications of DMFCs are an emerging area because DMFCs have low noise, thermal signatures and no toxic effluent. The DMFC relies upon the oxidation of methanol on a catalyst layer to form carbon dioxide. However, the electrooxidation of methanol often results in a variety of incomplete oxidation products such as formaldehyde (HCHO), formic acid (HCOOH), methyl formate (HCOOCH3), and carbon monoxide (CO). These byproducts could deactivate expensive cell catalysts and thus the fuel cell itself.
Therefore, it is highly desirable to have a fast, selective, and sensitive device to identify and quantify the fuel cell reaction products for a fundamental understanding of the reaction mechanism, as well as to screen the best catalyst and the reaction conditions. Many analytical techniques such as infrared spectroscopy, fluorescent spectroscopy, and electrochemical detectors have been employed to monitor the products resulting from methanol oxidation reactions, but these techniques lack detection selectivity. For example, differential electrochemical mass spectrometry (DEMS) has been adopted to probe fuel cell reactions; however, it only can detect volatile and small molecules such as CO2 and CO and is blind to HCHO and HCOOH, which cannot easily penetrate the membrane separating the electrochemical cell and the mass spectrometer chamber. In addition, recent efforts have been made to detect HCHO using electrospray ionization (ESI) mass spectrometry (MS) after online extraction of fuel cell products. However, the method involves complicated sample transfer lines and laborious extraction. Therefore, there is an urgent need for the development of analytic techniques that are fast, highly selective and sensitive to identify different cell reaction products, particularly those byproducts such as HCHO and HCOOH, in addition to CO2.
Desorption electrospray ionization (DESI) is a recent advance in the field of MS. DESI provides direct ionization of analytes with little or no sample preparation. Sample ionization by DESI occurs via the interactions with charged microdroplets generated in a pneumatically assisted electrospray of an appropriate solvent. In addition to analysis of solid samples, DESI has been extended to directly ionize liquid samples, and its demonstrated applications include the coupling of MS with chromatography, microfluidics, and electrochemistry, probing protein conformation, and developing submillisecond time-resolved MS.